1. Field of the Invention
This invention relates to a PWM/PFM control circuit and a switching power supply circuit and, more specifically, to those which are useful when improving efficiency under a light load of the switching power supply circuit.
2. Description of the Related Art
Among switching power supply circuits according to earlier technologies is that having a PWM/PFM control circuit which works under PWM control when load is a heavy load of a predetermined value or higher, and works under PFM control when the load is a light load of less than the predetermined value. An example of this type of switching power supply circuit is shown in FIG. 10. As shown in this drawing, such a switching power supply circuit is a combination of a chopper circuit, as an object to be controlled, and a PWM/PFM control circuit. When a switching element SW is turned on in the chopper circuit, current flows along a path, power source VIN→reactor L→switching element SW→power source VIN, and energy accumulated in the reactor L increases. When the switching element SW is turned off, the energy accumulated in the reactor L is released to the load side, and current flows along a path, power source VIN→reactor L→diode SD→capacitor C0 or load→power source VIN.
On the other hand, the PWM/PFM control circuit for exercising the on/off control of the switching element SW of the chopper circuit has a comparator 1 for comparing a reference voltage VREF with an output feedback voltage which is obtained by dividing the output voltage VOUT of the chopper circuit by resistances R1 and R2; a PWM control signal generator 2 for comparing an error signal S1, which is the output of the comparator 1 and represents a difference between both voltages, with a ramp signal S2 to output a PWM control signal S3; and a PFM control signal generator 3 for generating a PFM control signal S4, as a pulse signal turning the switching element SW on for a certain period of time, based on the PWM control signal S3 as the output of the PWM control signal generator 2.
The ramp signal S2 is obtained as an output signal of a triangular wave generator 5 based on a reference signal S6 which is the output of an oscillator 4. The PFM control signal S4 is formed based on the PWM control signal S3. A logic circuit 6 receives the PWM control signal S3 and the PFM control signal S4, and outputs a switch control signal S5 corresponding to the signal of the greater pulse width of the signals S3 and S4, thereby controlling the on/off state of the switching element SW. The logic circuit 6 comprises a NOR circuit 7 for adopting the nor logic of the PWM control signal S3 and the PFM control signal S4, and an inverter 8 for inverting the output of the NOR circuit 7. The switching element SW is formed from an N-channel transistor, and the gate of this transistor is supplied with the switch control signal S5.
FIGS. 11A to 11F are waveform charts showing the signal waveforms of the respective portions of the switching power supply circuit according to the earlier technology. FIG. 11A represents the output voltage VOUT, FIG. 11B represents the relationship between the error signal Si and the ramp signal S2, FIG. 11C represents the reference signal S6 as a reference for forming the PWM control signal S3 of a cycle T, FIG. 11D represents the PWM control signal S3, FIG. 11E represents the PFM control signal S4, and FIG. 11F represents the switch control signal S5.
As will become clear by reference to these charts, when the pulse width of the PWM control signal S3 formed from the error signal based on the output voltage VOUT and the ramp signal based on the reference signal S6 (the pulse width changes with load) is smaller than the pulse width of the PFM control signal S4 (the pulse width is constant), namely, under a light load, the switch control signal S5 based on the PFM control signal S4 is formed. When the load increases and the pulse width of the PWM control signal S3 becomes larger than the pulse width of the PFM control signal S4, the switch control signal S5 based on the PWM control signal S3 is formed.
As noted here, under light load, the pulse width of the PWM control signal S3 is small, and its oscillation is intermittent (at intervals based on the oscillation frequency during PWM action). The switching power supply circuit performs a PFM action of a varying frequency. Under a heavy load, a PWM action is performed in which the pulse width of the PWM control signal S3 is greater than the pulse width of the PFM control signal S4, and oscillation frequency is fixed. Under both conditions, the ripple voltage of the output voltage VOUT is low.
However, in a region where the PFM action is shifted to the PWM action, the PWM control signal S3 enters into a state where certain pulses of the oscillation frequency during the PWM action have been thinned out, posing the problem that the ripple voltage of the output voltage VOUT involves ripples of the oscillation frequency during the PWM action and great pulsations of a low frequency.
FIG. 12 is a characteristic chart showing the ripple voltage characteristics of the switching power supply circuit according to the earlier technology. With the above-described switching power supply circuit according to the earlier technology, as shown in the drawing, a high ripple voltage occurs in a transitional mode (in FIG. 12, a range with a load current of from 10 mA to 100 mA) in which changeover from the PFM action to the PWM action takes place.
Japanese Unexamined Patent Publication No. 1999-155281 discloses a means of obtaining efficiency under light load by changing the value of constant current to decrease the frequency of PWM control itself. However, as paragraph [0014] of this publication indicates, the above means encounters the problem that the ripple voltage increases.